How can I force (outgoing) connections to a certain hostname to be routed via certain WAN interface (Peplink 380)? For example, uploads to YouTube should always go via WAN1, as it has an upload speed of 10 Mbps (download 30 Mbps). But now it always goes via WAN2 (upload 5 Mbps; download 120 Mbps). Problem is, these youtube servers have hundreds of IPs, and I find no setting to filter all traffic to youtube.com via 1 interface. How can I do that?
YouTube is just one example.

The coming firmware 5.4 supports the outbound policy by domain name which fit your requirement. The firmware 5.4 is currently in beta status. If you are interest in the beta program, please refer the following URL for details.